1. Field of the Invention
The present invention relates to a light water moderation type nuclear reactor, more particularly to a pressurized water type nuclear reactor with a once-through method or a boiling water type nuclear reactor with a once-through method.
2. Description of the Prior Art
The method of utilizing the fuel materials in a light water moderation type nuclear reactor (hereinafter referred to as a light water reactor) is largely classified into the once-through method and the reprocessing and recycling method.
With the once-through method the light water reactor uses the enriched uranium and in this method none of the fuel materials contained in the used fuel rods which are taken out of the light water reactor is reused (recycled) in the light water reactor. This once-through method or system cycling cost when cost of the reprocessing fuel is higher than that of the enriching uranium.
Additionally, the purpose of the reprocessing and recycling method or system is to make new fuel rods by reprocessing fuel materials in the used fuel rods and to charge those new fuel rods into the light water reactor to reuse the fuel materials.
One method to effectively use the fuel materials by the once-through method is to greatly increase the take-out burnup from the fuel assembly, that is, to realize a high degree of the burnup. The fuel assembly includes many fuel rods. It is required to raise the enrichment of the enriched uranium so as to achieve the high degree of the burnup. However, to realize such a raised enrichment of the enriched uranium, the following problems occur.
In the center of the reactor core of the light water reactor there are the fuel assemblies with large difference in the neutron infinite multiplication factor because of a high enrichment of new fuel assemblies and the large take-out burnup. A difference in the output power share proportions of the individual fuel assemblies, therefore, the output power mismatch grains larger and also the output power peaking becomes larger.
Further, as the enrichment increases, the surplus reactivity which has to be controlled in the initial stage of the burning increases. Therefore, the conventional fuel assembly using the fuel rods containing gadolinia has to increase the number of the fuel rods which contain the gadolinia.
The fuel rods of reactor core of the conventional pressurized water type nuclear reactor have uniformly the ratio (r.sub.H/U) of the number of hydrogen atoms to the number of fuel material atoms of about 2.0. The characteristics of the reactor core of the conventional pressurized water type nuclear reactor is represented by the curve P.sub.5 (a dashed line) as shown in FIG. 11. The initial enrichment of the fuel rods of the conventional light water reactor is raised until the take-out burnup E.sub.b represented by the curve P.sub.5 as shown in FIG. 11 is realized.
With this reactor core the initial neutron multiplication factor is large, and in order to suppress this, a large amount of the burnable poison material such as gadolinium has to be put in the fuel assemblies at the expense of the neutron economy.
Furthermore, the mingling of the fuel assemblies, which are much different in the neutron multiplication factor, into the reactor core makes it difficult to flatten the output power distribution. The maximum burnup is restricted by the fuel rods having the peak power with the result of the lowered average take-out burnup.
The mismatch in the neutron multiplication factor of the conventional light water reactor is large, therefore the average take-out burnup from the fuel assembly can not be made high. The realization of the high degree of the burnup can not realize is impossible.
The conventional light water reactor has a uniform ratio (r.sub.H/U) (about 2.0) of the number of hydrogen atoms to the number of fuel material atoms in the reactor core.
The variation of the neutron multiplication factor in the conventional light water reactor is shown the curve P.sub.4 (a dashed line) shown in FIG. 10. In the conventional light water reactor the fuel assemblies are exchanged for the new fuel assemblies at the burnup E.sub.c. Namely the average take-out burnup of the fuel rods charged in the reactor core of the conventional light water reactor is the burnup E.sub.c. The amount of the charged fuel in the conventional light water reactor is the same throughout the reactor core.
The average take-out burnup E.sub.c of the fuel rods in the reactor core is not made larger, therefore the uranium saving can not be achieved in the conventional light water reactor.
From the standpoint of the effective use of the uranium resources, a light water reactor has been proposed in which the conversion from uranium-238 to a fissile product (plutonium-239) is improved.
In the "General Features of Advanced Pressurized Water Reactors with Improved Fuel Utilization" by Werner Oldekop et al in the Nuclear Technology, vol. 59, November 1982 P. 212-227, a high conversion reactor (HCR) was proposed to lower the ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material in the reactor core of the light water reactor from conventional 2.0 to 0.5 and raise the average energy of neutrons to make the plutonium conversion rate higher than 0.9. As a construction to bring about this ratio (V.sub.H /V.sub.F) of the volume of light water and the volume of fuel material of 0.5 a dense lattice construction is employed.
Because of the dense lattice construction, the high conversion reactor (HCR) has following serious problems therein raised from the aspects of the heat transfer or the floating. Such problems are as follows, for example, the pressure drop in the reactor core becomes about four (4.0) times as much as that of the conventional light water reactor, or by the unexpected accident with coolant loss the emergency coolant hardly enters into the reactor core.
The high conversion reactor (HCR) including this example aims at an effective utilization of the fuel material by reprocessing and recycling the fuel assemblies taken out of the reactor core. In the high conversion reactor (HCR) the fuel cycle including the steps of the fuel reprocessing, the fuel reworking, etc. must be completed.
Furthermore, even if the above described problems in the high conversion reactor (HCR) were solved therein, the utilization quantity of uranium in the high coversion reactor (HCR) may be not reach more than about two-and-a-half (2.5) times as much as that of the conventional light water reactor.